1. Field of the Invention
The present invention relates to a fixing unit for a copying machine and, more particularly, to a roller type fixing unit which has a pair of rollers for thermally fixing a thermoplastic toner image on a sheet passing between the pair of rollers.
2. Description of the Prior Art
A roller type fixing unit for a copying machine, which has a heating roller incorporating a heater and a pressurizing roller which is turned while being suitably pressed against the heating roller, is well known in the art. A toner image is thermally melted on a sheet and fixed to the sheet by allowing the sheet to pass between the heating and pressurizing rollers.
When a sheet or sheets are jammed in a copying machine which has this type of fixing unit, the supply of current is automatically interrupted so that the operation of the copying machine is stopped. Before the copying machine can be started again, the formed sheet or sheets must be removed. Access to the formed sheet or sheets is obtained through an opening and closing frame part of the copying machine, such as a cover or lid. However, when a sheet is jammed between the heating roller and the pressurizing roller, it is difficult to pull out the sheet because it is forcefully clamped between the two rollers. In practice, a large pressure on the order of 30 to 50 kg exists between these two rollers, and, if the formed sheet is forcibly pulled out from between the two rollers, the surface of the heating roller becomes dirty and is spoiled by the toner which has not yet been fixed to the formed sheet. Thereafter, as a result, a satisfactory fixing operation is not possible.
In order to eliminate these problems in the prior art, it has been proposed to equip a conventional copying machine with a mechanism which is used to disengage the heating roller and the pressurizing roller from each other when a sheet is jammed in the copying machine. The mechanism is designed so that an electrical signal, produced when a sheet is jammed, is utilized to actuate a mechanical operation to disengage the heating roller and the pressurizing roller. However, since such a mechanism requires an intricate control circuit, its manufacturing cost is, accordingly, high, and its reliability is poor.
In another prior art copying machine, the two rollers are manually disengaged from each other. However, since the operator can forget to engage the rollers with one another after the formed sheet has been removed, this method also has disadvantages.
In order to eliminate the above-described difficulties, the present Applicant has proposed, in Japanese Patent Application No. 53738/1977, a roller type fixing unit for a copying machine in which the motion of the copying machine lid, which is necessarily opened and closed in order to remove the jammed sheet, is utilized to engage and disengage the pair of rollers.
In Applicant's prior art roller type fixing unit, one of the rollers is urged to swing upwardly by a spring force, and this roller then swings downward against the spring force when the lid is closed so that the first roller is brought into contact with the other roller. Therefore, when the lid is closed, the spring force acts as a direct load resistance, i.e., it is difficult to close the lid.